Hydrogen and helium constitute ninety-nine percent of the matter in the solar system. The helium-to-hydrogen ratio of the material from which the solar system was formed is an essential ingredient in all stellar evolution calculations. It reflects the conditions of the first few minutes of the "big bang" and forms a boundary condition for modeling the evolution of our own star. One way to determine this ratio is to study the sun because it contains most of the mass of the solar system. However all measurements are quite indirect and subject to uncertainties in interpretation. The giant planets, particularly Jupiter, should be composed of original solar nebula material and are considered to be good indicators of the initial solar system composition. Thus measurement of helium to hydrogen ratios is an extremely important objective of any mission to the outer planets.
A number of techniques have been considered for making helium to hydrogen ratio measurements. These techniques include mass spectrometry, gas chromatography and differential pressure measurements. Alpha scattering was used to measure chemical composition of lunar material by unmanned lunar landing spacecraft. However these alpha scattering measurements were based on back scattering of alpha particles in which the lunar soil formed a thick target. Observation of scattered alpha particles at backward angles allowed maximum separation of detected materials. This previously utilized technique is unsuitable for helium and hydrogen measurements because their scattering products occur in a forward hemisphere.